Functionally gradient polymeric materials

ABSTRACT

Functionally gradient polymeric materials are formed by a process that utilizes an ascending polymerization front. A polymerization front is initiated in a reaction vessel containing a monomer solution. Additional monomers or resins are added on top of the polymerization front to maintain a substantially constant level. The composition of the monomers and/or additives are varied as they are added to the reaction vessel to form a functionally gradient polymeric material.

This is a continuation of co-pending application Ser. No. 09/128,126filed Aug. 3, 1998, now U.S. Pat. No. 6,057,406.

BACKGROUND OF THE INVENTION

This invention was made with Government support under ContractF149620-94-1-0459P0001 awarded by the Air Force Office of ScientificResearch and Contract CTS-931917 awarded by the National ScienceFoundation. The Government has certain rights in this invention.

The present invention relates to polymeric materials and composites madeby frontal polymerization More particularly, the present inventionrelates to functionally gradient polymers formed by frontalpolymerization.

Functionally gradient or graded materials (FGMs) are materials whosecomposition varies spatially in a controlled manner. Many differentmethods have been devised for forming functionally gradient materials.In one such process, centrifugal force was used to prepare gradients ina carbon fiber reinforced epoxy composite to produce composites withspatially varying conductivity. Additionally, several researchers havedone work on preparing gradient interpenetrating polymer networks(IPNs). Most of the work developed by these individuals involvesproducing a gradient by diffusing one component into another pregelledcomponent followed by curing, or producing a gradient in the polymerusing a gradient of illumination. The diffusing method can require asmuch as 280 hours to produce a gradient over 10μ. Using the absorptionof light to produce a gradient is limited to polymers with a thicknessless than 1 mm. None of these techniques can be used to producegradients in polymers which are several centimeters in thickness.

Graded polymeric materials, such as Graded Refractive Index (GRIN)materials have found wide use in optical applications. These materialsare prepared via interfacial gel polymerization, which is a slow processlimited to producing gradients less than about 1 centimeter.

Another type of gradient material with definite utility is an opticallimiter based on a gradient of nonlinear optical dye dissolved in apolymer matrix. An optical limiter is a device that strongly attenuatesintense optical beams but allows high transmittance at low level light.Such a device would be very useful for protecting human eyes fromintense laser pulses. A discussion of the types of organic materialsthat exhibits such nonlinear absorption is contained in Perry et al.,“Organic Optical Limiter with a Strong Nonlinear Absorptive Response.”Science, 1996, pages 1533-1536. They found that metallophthalocyanine(M-Pc) complexes containing heavy central atoms work well. These dyesare compatible with poly(methyl methacrylate) and dissolve in themonomer. This affords the great advantage of inexpensive materials.

Frontal polymerization is a method for converting monomer into polymervia a localized reaction zone that propagates through the coupling ofthe heat released by the polymerization reaction and thermal diffusion.Frontal polymerization was first discovered at the Institute of ChemicalPhysics in Chemogolovka, Russia by Chechilo and Enikolopyan in 1972.Polymerization fronts can exist with free-radical polymerization ofmono- and multifunctional acrylates or epoxy curing. Frontalpolymerization can be achieved in solution polymerization with monomerssuch as acrylamide, methacrylic acid and acrylic acid in solvents suchas water and DMSO.

Frontal polymerization reactions are relatively easy to perform. In thesimplest case, a test tube is filled with the reactants. The front isignited by applying heat to one end of the tube with an electric heater.The position of the front is obvious because of the difference in theoptical properties of polymer and monomer. Under most cases, a plot ofthe front position versus time produces a straight line whose slope isthe front velocity. The velocity can be affected by the initiator typeand concentration but is on the order of centimeters per minute.

The defining feature of thermal frontal polymerization is the sharptemperature gradient present in the front. The temperature can jumpabout 200° C. over a little as a few millimeters, which corresponds topolymerization in a few seconds at that point.

In view of the foregoing, it would be a significant advancement in theart to provide a process for forming functionally gradient polymerswhich had a short reaction time and which could produce polymers severalcentimeters in thickness. Such a process and the polymeric materialscreated thereby are disclosed herein.

BRIEF SUMMARY OF THE INVENTION

The present invention is directed to functionally gradient materials anda process for forming the same. In a preferred embodiment, an ascendingpolymerization front is created in a reaction vessel. Pumps providemonomers or resins in a controlled ratio on top of the ascending frontas it propagates to maintain a nearly constant thickness of unreactedmonomers. The height of unreacted monomers is maintained such that thefront is not extinguished, but progresses in a controlled manner. Byvarying the ratio of the monomers and/or the concentration of additivesto the mixture, functionally gradient materials can be formed.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is directed to functionally gradient materials andmethods for forming them.

As discussed above, frontal polymerization reactions are generallyrelatively easy to perform. In the simplest case, a test tube is filledwith reactants and a front is ignited ii by applying heat to one end ofthe tube. The front then either ascends up the tube or descends down thetube, depending upon the point at which it was initiated. In a preferredembodiment of the present invention, the process is carried out in acylindrical tube such as a test tube. However, it should be appreciatedthat reaction vessels having different cross-sectional areas can also beused.

In the preferred embodiments, the cross-sectional area can vary fromabout 15 mm² to about 1 m². The limiting factors on the upper end of thearea are dependent in part upon the ability to simultaneously addadditional reactants to the entire surface area and the ability of thefront to progress in a uniform manner.

According to the preferred embodiment of the present invention, thepolymerization front is an ascending front in a vertical container.However, it will be appreciated by those skilled in the art that otherconfigurations can also be used without departing from the spirit orscope of the invention. For example, the reactor may be tilted orpossess a varying cross section.

Frontal polymerization is often carried out at ambient temperature withthe heat of reaction being sufficient to sustain the reaction as thefront progresses through the material. However, in some systems it maybe necessary to preheat the reactants or to provide additional localizedheat as the front progresses to maintain the polymerization reaction.

The process of the present invention can be utilized to make manydifferent types of functionally gradient polymers. In one preferredembodiment, the functionally gradient polymer comprises a polymer matrixhaving an optical dye dissolved therein. The concentration of the dyevaries along the length of the polymer sample. This is achieved byvarying the amount of dye added to the monomer that is added to the topof the polymerization front. It will be appreciated by those skilled inthe art that there are many different ways of varying the concentrationof the dye. For example, the dye can be added by a separate pump as themonomer is being added such that its concentration gradually changes, byvarying the relative flow rates of the two feedstreams. Alternatively,the dye solution can be premixed with the monomer in the inlet reservoirby a separate pump such that the inlet stream's dye concentration varieswith time.

Many different types of dyes and additives can be used in the presentinvention. Examples of additives include plasticizing agents, rubbertoughening agents and inert fillers. The latter can be affected to greatadvantage because the rapid reaction in the front prevents sedimentationof the filler. For example, diethyl phthalate can be used to prepare agradient of plasticization. Rubber particles (from ground car tires) orpoly (butadiene) can be used as rubber toughening agents. Silica gel orcalcium carbonate can be used as inert fillers.

In another preferred embodiment of the present invention, functionally(gradient polymers in which the mechanical properties of the polymervary along the length are formed. In one embodiment, the mechanicalproperties of the polymer are modified by varying the amount ofcross-linking that occurs in the polymer. This can be achieved bychanging the ratio of monomers that are added to the system or byvarying the amount of cross-linking agent that is added to the system.Examples of suitable systems include multifunctional acrylates with amonoacrylate and/or an epoxy resin with its curing agent. Polymericmaterials with a gradient in the copolymer composition can be achievedby varying the comonomer feedstreams.

The invention can be further understood by reference to the followingexamples:

EXAMPLE 1

Tri(ethylene glycol)dimethacrylate (TGDMA) and benzyl acrylate were usedas monomers in a polymerization reaction. They were stored overmolecular sieves dried over CaH₂ and then filtered before use. Lupersol231 was used as an initiator. A peristaltic pump was used to supply themonomers into a test tube having an inner diameter of 22 millimeters inwhich an ascending front was propagated. The polymerization reaction wasinitiated by heating the bottom of the test tube with an electricheater. The ratio of the monomers was varied from 100% TGDMA/0% benzylacrylate at the beginning to 5% TGDMA/95% benzyl acrylate at the end.The sample diameter was 2.2 cm and its length 10 cm. The flow rate ofmonomers into the test tube was 3 mL/minute.

The characteristics of the resulting functionally gradient polymer werea rigid material at the high TGDMA end smoothly graded into a rubbery,crosslinked material at the low TGDMA concentration.

EXAMPLE 2

Tri(ethylene glycol)dimethacrylate (TGDMA) was used as the monomer in apolymerization reaction. Tricaprylmethylammonium persulfate was used asan initiator in this experiment producing dye gradients. The mainadvantage of this initiator is its gasless nature under decomposition.This permitted the formation of bubble-free optical materials.

Aluminum phthalocyanine chloride was used as a dye. The dye wasdissolved in the TGDMA, and the solution was used as a coloringcomponent. A peristaltic pump was used to supply the reactive materialinto a test tube in which an ascending front was propagated. The innerdiameter of the test tube was 12 mm. The test tube was exposed toambient pressure and temperature conditions. The dye concentration wasgradually increased in the TGDMA reservoir as the monomer was added tothe system until it reached its saturation point.

The dye concentration varied in the final sample from saturated (darkgreen) to zero over a distance of 5 cm.

The process of the present invention can be utilized in many differenttypes of polymer systems such as reactive acrylates, acrylamides insolution, and cationically and amine cured epoxies. The process willwork with essentially any polymer system that will support frontalpolymerization. It can be used for graded IPNs using binary frontalpolymerization as well as graded rubber toughened epoxies using frontalcuring.

While the present invention has been described with respect to thepresently preferred embodiments, numerous changes and substitutions canbe made to the products and processes of the present invention withoutdeparting from the scope of the invention. Accordingly, all changes ormodifications which come within the meaning and range of equivalency ofthe claims are to be embraced within their scope.

What is claimed is:
 1. A functionally-gradient polymeric material formedby frontal polymerization comprising the steps of: initiating apolymerization front in a liquid monomer solution in a reaction vesselsuch that the front progresses generally linearly through the reactionvessel; adding additional monomer solution to the reaction vessel aheadof the front; and varying the composition of the monomer solution as itis added to the reaction vessel.
 2. A functionally gradient polymericmaterial as defined in claim 1 further comprising a plasticizing agent.3. A functionally gradient polymeric material as defined in claim 1further comprising a rubber toughening agent.
 4. A functionally gradientpolymeric material as defined in claim 1 further comprising an inertfiller.